On 01/24/18 13:16, Frank Rowand wrote: > On 01/24/18 07:47, Rob Herring wrote: >> On Tue, Jan 23, 2018 at 3:17 PM, Frank Rowand <frowand.list@xxxxxxxxx> wrote: >>> On 01/23/18 04:42, David Gibson wrote: >>>> On Mon, Jan 22, 2018 at 03:01:52PM -0600, Rob Herring wrote: >>>>> On Mon, Jan 22, 2018 at 2:09 AM, Frank Rowand <frowand.list@xxxxxxxxx> wrote: >>>>>> Hi All, >>>>>> >>>>>> I've tried to create a decent distribution list, but I'm sure I've missed >>>>>> someone or some important list. Please share this with anyone you think >>>>>> will be affected. >>>>>> >>>>>> I have been playing around with some thoughts for some additions to >>>>>> the devicetree FDT aka blob format. >>>>>> >>>>>> I would like to get the affected parties thinking about how additions to >>>>>> the format could improve whichever pieces of FDT related technology you >>>>>> work on or care about. In my opinion, the FDT format should change >>>>>> very infrequently because of the impact on so many projects that have >>>>>> to work together to create a final solution, plus the many many users >>>>>> of those projects. >>>>> >>>>> A few things discussed before: >>>>> - Adding type information Even just tagging phandles would be good. >>>> >>>> I'm a bit dubious about this. It would have to be "hints" only - >>>> there's not really anyway we can put in authoritative type >>>> information, since dtc itself doesn't really know that. It's also >>>> hard to see how that could be done in a way which wouldn't either a) >>>> require very awkward parallel lookup of the data and type information >>>> or b) not be backwards compatible (even read only). >> >> I never said it was possible. :) I'm just trying to enumerate possible >> FDT format changes because I don't think we want to continuously >> trickle out FDT changes even if they are backwards compatible. > > Yes, I'm trying to capture any pending changes in a single version change. > > >>>>> - Allow applying overlays by just appending to the blob. The need for >>>>> this is somewhat gone now that libfdt can just fully apply overlays. >>>> >>>> I'm not really sure what you want here. I mean you could easily allow >>>> the format to allow multiple appended overlays, and define that to >>>> mean the overlaid result. At some point *something* is going to have >>>> to really do the application, so I'm not sure that it really buys you >>>> that much. It also makes nearly every operation on the tree in libfdt >>>> horrible to implement, at least within the other constraints the >>>> interface was designed around; you'll continually have to scan the >>>> entire tree just in case some other overlay fragment has some bearing >>>> on the thing you're looking at. It confuses the interface too: what >>>> does "node offset" mean if the same node could be built up from >>>> overlay fragments at multiple offsets. >> >> The idea was to avoid applying overlays to flattened trees at all. >> You're just passing the problem to the next stage (typically the >> kernel). But we have applying overlays to flattened trees now, so >> maybe there's no need anymore. >> >>> Somewhat echoing David's comment, I'm also not sure what you mean. >>> And trying to not overly influence this conversation with preconceptions >>> from what I'm going to propose. >>> >>> My first shot at the new format added a field to the FDT to indicate >>> that an overlay FDT was concatenated to the FDT (and the overlay FDT >>> in turn could set it's field to concatenate another overlay FDT). >> >> Yes, something like this is what I meant. This was something Grant had >> talked about. >> >>> But in the end I decided that information belonged outside the FDT, >>> and it was sufficient to require that all FDTs be padded out so that >>> if an overlay FDT was concatenated to the FDT, the overlay FDT would >>> be properly aligned. >> >> I can't think of why this wouldn't work either. >> >>> For ease of typing, I'll call this "chaining" or "chained". For >>> Linux, I am envisioning no kernel use of data from a chained FDT >>> until after the tree is unflattened. I haven't done an exhaustive >>> search to determine all of the uses of data directly from the >>> flattened FDT, but I am optimistic that there will not be any such >>> access that would require data from a chained FDT (and we could >>> make this a rule). >> >> This would be a major downside to leaving it up to the kernel because >> what can't be touched is hard to enumerate and could change. For >> example, we added earlycon and now the uart node can't be modified. > > What you say makes sense. So I'll reverse myself and say that for > Linux, we should update the FDT read code to scan all chained > overlay FDT(s) as well as the base FDT. < snip > What I wrote was somewhat ambiguous. What I meant by "FDT read code" was functions that check for existence of nodes in the FDT or read property values from the FDT. The other way one could read what I wrote was that when Linux unflattens the FDT, it would unflatten the FDT and all of the chained FDTs. Obviously also true, but not what I was trying to say here. -Frank -- To unsubscribe from this list: send the line "unsubscribe devicetree-spec" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
